I found that exercise to be hit or miss. I think an easy fix is to try deleting two cities then search for the city named flag. I thought that worked for me when I did it, but its been a little bit.
-onthesauce
You really have to look around to solve it since the question is very confusing but in general it is easy since you have to do it like this:
‘{“city_name”:“flag”,“country_name”:“”}’, because you have to leave the country_name empty since this is the one that stores the flag and I think that is where people make mistakes